1. Field of the Invention
The invention relates to a structure and control method of a vehicular belt-driven continuously variable transmission. More particularly, the invention relates to the structure and control method of a vehicular belt-driven continuously variable transmission that eliminates a centrifugal hydraulic pressure canceller chamber of a secondary side cylinder.
2. Description of the Related Art
One known type of vehicular transmission is a belt-driven continuously variable transmission that shifts speeds smoothly and continuously without any gear switching. This belt-driven continuously variable transmission is formed of a continuously variable transmitting portion that mainly includes two rotating members arranged parallel to one another, a primary pulley provided on one of the rotating members so as not to be able to rotate relative to that rotating member, a secondary pulley provided on the other rotating member so as not to be able to rotate relative to that rotating member, and a belt that is wound around the two pulleys. The primary pulley and the secondary pulley each include a fixed sheave and a movable sheave, with a V-shaped groove in which the belt sits formed between the two. Power is transferred between the two pulleys via the belt. Here, a primary side cylinder, which applies thrust for moving the movable sheave of the primary pulley in the axial direction, is provided on the primary pulley, while a secondary side cylinder, which applies thrust for moving the movable sheave of the secondary pulley in the axial direction, is provided on the secondary pulley. By individually controlling the hydraulic pressure supplied to the primary side cylinder and the secondary side cylinder, the speed ratio of the belt-driven continuously variable transmission is changed by controlling the groove width of the primary pulley and changing the winding diameter of the belt around that pulley, while belt tension is controlled by changing the groove width of the secondary pulley.
In this kind of belt-driven continuously variable transmission, when rotation from a power source such as an engine is input to a continuously variable transmitting portion without a reduction in speed while the vehicle is traveling forward, the rotational speed of the secondary pulley increases, and as it does so, relatively large centrifugal hydraulic pressure is generated within the secondary side cylinder. This centrifugal hydraulic pressure applies thrust to the movable sheave of the secondary pulley in a direction that squeezes the belt such that the belt squeezing force becomes excessive. Because of this, one related belt-driven continuously variable transmission is provided with a centrifugal hydraulic pressure canceller chamber on the secondary pulley side to cancel out the centrifugal hydraulic pressure.
FIG. 6 is a sectional view of a secondary pulley 200, which is a constituent member of the foregoing related belt-driven continuously variable transmission. The secondary pulley 200 includes a fixed sheave 204 integrally provided on an output shaft 202, a movable sheave 206 fitted onto the output shaft 202 so as to be able to move in the axial direction but not rotate relative to that output shaft 202, and a secondary side cylinder 208 provided adjacent to the movable sheave 206. The secondary side cylinder 208 has a hydraulic pressure chamber 212 formed by the movable sheave 206 and a partition 210, and a centrifugal hydraulic pressure canceller chamber 214 formed between the partition 210 and a peripheral wall 213 that is fixed to the movable sheave 206. That is, the centrifugal hydraulic pressure canceller chamber 214 is formed on the opposite side of the partition 210 from the pressure chamber 212. By providing the centrifugal hydraulic pressure canceller chamber 214, a centrifugal hydraulic pressure equal to that in the hydraulic pressure chamber 212 is generated in the centrifugal hydraulic pressure canceller chamber 214 against the thrust on the movable sheave 206 toward the fixed sheave 204 which is generated by the centrifugal hydraulic pressure that is generated as the hydraulic pressure chamber 212 rotates. The centrifugal hydraulic pressure generated in this centrifugal hydraulic pressure canceller chamber 214 suppresses the effect of the centrifugal hydraulic pressure generated in the hydraulic pressure chamber 212 by applying thrust to the movable sheave 206 which is in the opposite direction as the thrust generated by the centrifugal hydraulic pressure in the hydraulic pressure chamber 212.
Providing this centrifugal hydraulic pressure canceller chamber however makes the continuously variable transmitting portion heavier, less compact, and more expensive. Therefore, Japanese Patent Application Publication No. JP-A-2005-90719 describes technology which eliminates this centrifugal hydraulic pressure canceller chamber by forming the secondary side cylinder with two hydraulic pressure chambers, i.e., an outer diameter side hydraulic pressure chamber and an inner diameter side hydraulic pressure chamber, and appropriately switching the cylinder pressure receiving area.
However, with the technology described in JP-A-2005-90719, a structure is necessary to switch between a mode that supplies hydraulic pressure to the inner diameter side hydraulic pressure chamber and discharges hydraulic pressure from the outer diameter side hydraulic pressure chamber, and a mode that supplies hydraulic pressure to the inner diameter side hydraulic pressure chamber and also supplies hydraulic pressure to the outer diameter side hydraulic pressure chamber. However, the required structure is rather complex. Also, the belt squeezing force does not change smoothly as the cylinder pressure receiving area, which is related to the switching of the modes, is switched.